1. Technical Field
The present invention relates to an illuminance acquiring device, an illuminance acquiring method, and an illuminance acquiring program which acquire an intensity of light sensed by an image pickup device for converting the intensity of light into electric energy.
2. Related Art
A high-quality image display can be realized by exactly performing an image evaluation process and performing an image adjustment process in which the evaluation result is reflected. It is important to acquire brightness and color tone of a display screen of a display device with high accuracy and precision and various technologies therefor are suggested.
A device acquiring the brightness or color tone of the display screen of the display device acquires an image which the display device displays under an experimentally determined measurement condition by using various types of sensors such as an accurately corrected brightness meter, a color meter, and a charge coupled device (CCD) camera and acquires the brightness and color tone of the display screen of the display device by property processing output data.
However, the acquiring device using such a method cannot acquire the output data in which the recognizable brightness or color tone is reflected with high precision due to an influence such as a resolution of an intensity of light by a sensor or a noise generated in the sensor. Additionally, it is considered that a repeatability of the output data is not enough as well.
In order to perform a higher precision measurement, there is a device which acquires the display screen by cooling a temperature of the sensor lower than the normal temperature and by suppressing a sensor noise. However, there are various problems that a large-scale device is necessary, a price of the device is high, and an operational environment or condition of the device is limited, etc.
A method that the light energy is acquired by using a relation table that represents a correspondence relation between an output value of a CCD sensor and a product intensity of light×exposure time=light energy) of the intensity of light sensed by the CCD sensor and an exposure time by using the image pickup data which the CCD camera obtains by picking up the display screen of the display device while changing the exposure time; and the intensity of light (also called a radiant illuminance) sensed by the CCD sensor from the acquired light energy and the exposure time is conceivable. The relation table representing the correspondence relation between the output value of the CCD sensor and the product (light intensity×exposure time=light energy) of the intensity of light sensed by the CCD sensor and the exposure time is called the relation table between the output value of CCD sensor and light energy.
However, since the light energy is acquired by using the output value of the CCD sensor n which an influence of a noise generated in the CCD sensor is not considered and the radiant illuminance is calculated from the acquired light energy, the influence of the noise is involved in the calculated radiant illuminance. Accordingly, the described method has a problem that the calculated radiant illuminance is changed by a method of setting the exposure time.
For example, a technology synthesizing a plurality of image data in which an exposure condition is different and generating the image data with the high dynamic range is disclosed in JP-A-7-131708. When a plurality of image data in which the exposure condition varies are synthesized and the image data with the high dynamic range are generated, the technology disclosed in JP-A-7-131708 recalculates a conversion equation (magnification ratio) for converting the image data into the image data with the high dynamic range whenever synthesizing the image data.
That is, the technology disclosed in JP-A-7-131708 synthesizes the image data with the high dynamic range without an exposure-excess area or an exposure-shortage area by calculating each pixel value of the image data with the high dynamic range from the exposure time and the pixel value of the image data. In this case, the conversion equation (magnification ratio) for converting the image data into the image data with the high dynamic range is recalculated whenever synthesizing the image data.
However, since the noise generated in the image pickup device such as the CCD sensor is not considered in the technology disclosed in JP-A-7-131708, there is a problem that a value of the image data with the high dynamic range is considerably changed by the method of setting the exposure time.
A gist of the technology disclosed in JP-A-7-131708 is not to acquire the brightness or color tone of the display screen for performing the image evaluation process of the display device, but mainly to expand the dynamic range of a digital camera and the like using the image pickup device such as the CCD sensor. Accordingly, it is understood that it is not nearly necessary to consider the noise generated in the image pickup device such as the CCD sensor. However, in order to accurately perform the image evaluation process of the display device, when the gist is to acquire the brightness, the color tone and the like of the display screen, it is important that the noise generated in the image pickup device such as the CCD sensor is considered.